Minute virus of mice (MVM) is an autonomous parvovirus, one of a group of small single-stranded DNA viruses that are pathoengic for many vertebrate species, including man, and exhibit teratogenic, immunosuppressive and tumor suppressive activity in vivo. The efficient expression of the viral genome on cellular functions expressed transiently during S-phase of the cell cycle and on intracellular, differentiation-specific host cell factors. Our specific aims are to: 1) Precisely define the temporal order of appearance of viral macromolecules during infection of tightly synchronized cells and to examine the role of the MVM large non- structural protein (NS-1) in this process using our newly constructed NS-1 temperature-sensitive mutation; 2) Isolate and characterize additional mutations in the MVM non-structural genes and characterize second-site suppressors of our existing temperature-sensitive NS-1 mutant; and, 3) Characterize the differentiation stage-sensitive element within the capsid coding open reading frame that is important for the efficient expression of the MVM genome in fibroblasts. These experiments are designed to more precisely clarify how the lytic cycle of MVM is regulated. Because of the extreme dependence of the autonomous parvoviruses on intracellular host factors expressed transiently during S-phase and at specific stages of differentiation, they are ideal probes for analyzing the cellular proteins and regulatory signals normally used by the host cell during its replication and developmentally regulated gene expression. The long-term objective of these experiments is to gain significant insight into these cellular processes.